1. Field of the Invention
The present invention relates to a method and apparatus for controlling the coating thickness of materials applied to the surface of semiconductor wafers. More particularly, the present invention relates to a method and apparatus for controlling the thickness and uniformity of thin films deposited upon the surface of semiconductor wafers by spin coating deposition.
2. Background and Related Art
Spin-on deposition methods, as known in the semiconductor manufacturing art, are used to form thin films on semiconductor devices, such as, semiconductor wafers. Typically, a solution comprising at least one solvent and a precursor of the material to be deposited is placed in the center of a semiconductor wafer and then the wafer is rotated at a rate sufficient to distribute the solution across the surface of the wafer. The amount of solution, the solution viscosity, the solvent evaporation rate and rotational speed determine, in general, the thickness of the spin-on coating.
The spin-on deposition method of depositing thin films is used in the formation of a variety of materials. For example, spin-on deposition may be used to form photoresist layers and various insulating layers, such as, low-K dielectric polymer layers. In a conventional spin coating process, the semiconductor wafer to be processed is placed on a rotable chuck and held in place by a vacuum. However, as wafers have become increasingly larger, it has been found that the chucks' ability to hold the wafer in place at higher rotational speeds is diminished. This limitation on rotational speed has created difficulties in controlling the thickness of the deposited layer.
For example, in the deposition of low-K dielectric polymer materials, this speed limitation necessitates the manufacture of several grades of the precursor material to satisfy the various thickness requirements for the different wiring levels for device manufacture, such as, those employed in CMOS technologies. Each of the different grades has a different viscosity that limits thickness range. To obtain a required thickness within this range, the material is modified by adjusting the precursor/solvent ratio and then formulating new rotational spin speeds recipes to cover the thickness range. The difficulty with the above approach, however, is it is complex, costly and does not always give the results desired. For example, faster spin speeds are required to achieve the lower end of the thickness range and these faster speeds may result in planarity-related defects. It is also costly from a materials and process standpoint to have several grades of material for one or more interconnect technologies. In addition, dilution of the precursor material has to be performed manually, often necessitating several iterations to achieve acceptable wetting characteristics. This is time consuming and costly.